Patients with the Adult Respiratory Distress Syndrome of sepsis appear to exhibit an abnormal dependence of O2 uptake (VO2) on O2 delivery (blood flow x arterial O2 content). This "pathological O2 supply dependency" is manifested by an inability to adjust tissue A-V O2 extraction ratios in response to changes in O2 delivery. During progressive reductions in O2 delivery in animal models of endotoxemia or bacteremia, tissue VO2 becomes limited by O2 delivery (supply-limited) when O2 extraction is still low, suggesting a failure in tissue mechanisms regulating the distribution of a limited O2 supply. We hypothesize that this failure arises from a dysfunction in microvascular contractile, and/or endothelial-mediated relaxation responses. This may contribute to an impaired regulation of capillary recruitment, and/or regulation of transit time heterogeneity in capillaries and conducting vessels. Such a failure could explain the endotoxin-impaired oxygen extraction ability, by allowing the development of poorly perfused, anoxic tissue regions while other regions are over- perfused. The above hypothesis is supported by the in vitro observations that (a) stimulated contraction of isolated vascular rings from endotoxin- challenged animals is attenuated, and (b) that endothelial-dependent relaxation of precontracted vessels is impaired. Proposed studies will quantify the contributions of altered capillary recruitment and microvascular transit time heterogeneity adjustments in anesthetized animals challenged with E. coli endotoxin. Isolated intestine and skeletal muscle responses to graded reductions in O2 delivery at constant O2 demand, or stimulated increases in O2 demand at constant oxygen supply, will be measured. The significance of these responses for the maintenance of tissue VO2 will also be quantified. The role of endothelial-mediated relaxation in the microvascular regulation of normal tissues will also be determined, as will the significance of this for oxygen extraction ability. Finally, the consequences of reversing the endotoxin-induced vascular contractile defect by inhibiting vascular myocyte synthesis of nitric oxide, in terms of its effect on microvascular regulation and oxygen extraction ability, will be quantified. These studies will be carried out in using indicator dilution methodology (analyzed by two independent algorithms), and independent confirmation will be obtained using stereological morphology. Collectively, these studies will provide new information relating in vitro changes in intrinsic responsiveness of blood vessels to in vivo changes in microvascular regulation during endotoxemia, and the significance of this for tissue gas exchange and metabolism.